The present invention relates to actuators and fluid delivery systems, and especially, to nonmagnetic actuators and fluid delivery systems using such actuators suitable for use in the vicinity of a magnetic field of a magnetic resonance imaging system.
Magnetic resonance imaging (MRI) is used to image the body in a non-invasive manner. There are three types of electromagnetic fields used in MRI: a main static magnetic field (having field strengths from, for example, approximately 0.2 to several Tesla) which is generally homogeneous in the imaged volume; time varying magnetic gradient fields (Gx, Gy and Gz), which have different orientations and operate at frequencies of the order of 1 kHz; and a radio frequency (“RF”; having, for example a frequency of approximately 63.87 MHz at 1.5 Tesla).
MRI is often used to image patients that may be attached to other types of equipment, such as ventilators, infusion pumps, or other devices. Some of these devices fail to operate correctly in the high magnetic fields generated in MRI and/or create undesirable artifacts in the resultant image. As a result, there are a substantial number of MRI procedures that are delayed or canceled because the patient cannot be connected to the needed equipment during the MRI procedure. A review of issues related to the compatibility of various equipment in an MRI environment is set forth in Keeler, E. K. et al., “Accessory Equipment Considerations with Respect to MRI Compatibility,” JMRI, 8, 1 (1998), the disclosure of which is incorporated herein by reference. See also, Lemieux, L. et al., “Recording of EEG During fMRI Experiments: Patient Safety,” MRM, 38, 943 (1997).
For example, devices that contain electric actuators such as DC brush motors, step motors, brushless DC motors or other wound coil motors and solenoids often fail in a strong magnetic field as a result of damage to internal permanent magnets. Moreover, currents induced within the field windings of such devices by electromagnetic fields can cause overheating and potential damage to the windings and any connected electronic circuitry.
Furthermore, differences in magnetic permeability of materials within the actuator and eddy currents induced within actuator windings can affect the homogeneity of the MRI magnetic field, generating image artifacts. Actuators that use mechanical commutation, such as DC brush motors, can also generate radio frequency energy during switching which can induce unwanted artifacts upon the acquired MRI images.
To prevent damage to sensitive equipment in MRI procedures, U.S. Pat. No. 4,954,812 discloses a magnetic field alarm indicator to detect when the ambient magnetic field reaches unacceptable levels for equipment operation. After an alarm indication, the equipment can be moved farther from the MRI magnet or disconnected from the patient. An alarm indication can be ineffective, however, if the equipment must be placed physically close to the patient, such as for fluid administration, or if the equipment must be closely connected to the patient. The use of a magnetic field alarm indicator also does not address the problems of unwanted effects on magnetic field homogeneity and commutation or switching artifacts.
A number of medical devices have been designed to operate within the relatively high magnetic field environment used for MRI. For example, U.S. Pat. No. 5,494,036, discloses an injector system that provides for decreased interference between the magnetic field used for producing diagnostic images and the magnetic fields generated by the electric motors used for driving the pistons of the contrast media injectors.
Japanese Patent Application HEI 7-178169 and German Patent Application DE 197 14 711 A1 disclose use of a piezoelectric-based actuators such as ultrasonic motors in an MRI environment in an effort to reduce the adverse effects experienced with other actuators. Piezoelectric-based motors provide a means for generating oscillating movement electrostatically rather than magnetically when a changing excitation voltage is applied. The oscillating movement is then converted into rotary or linear motion depending on the motor mechanism, for example, a rotary friction drive for a rotary motor. Piezoelectric actuators have a number of inherent disadvantages in that they are fragile, do not generate large amounts of force, do not operate at high speeds, and often require complex electronic circuitry to provide the driving signals for the piezoelectric element(s).
A number of non-magnetic actuators materials have also been used in environments other than an MRI environment. For example, U.S. Pat. No. 5,919,167 discloses a syringe based fluid delivery device based on a shape memory alloy (SMA) activating element. U.S. Pat. No. 4,731,069, discloses a tube for use in a flow controller that employs a shape memory alloy control element to adjust the rate of flow of the fluid. U.S. Pat. No. 4,665,334 discloses a mechanism for generating rotary motion using a shape memory alloy metal actuator.
U.S. Pat. No. 5,630,709 discloses the use of a piezoelectric stack and an electrostrictive material (magnetostrictive) as electroactive actuators to drive pistons and valves within an in-line pump with a transverse piston displacement arrangement. U.S. Pat. No. 5,336,057 discloses a pump that includes a liquid-absorptive polymer gel as part of an actuator for discharging liquid from a tank. The polymer gel operates by expanding when liquid is absorbed and is not electroactive.
It is very desirable to develop actuators and fluid delivery systems using such actuators for use in an MRI and other environments that reduce or eliminate the problems with current devices discussed above.